In such reactors, water, the purpose of which is to function as coolant and moderator in the nuclear reactor in the nuclear plant, circulates. It is important to keep this water clean from debris. If debris particles are allowed to accompany the water in the core of the reactor, these may cause defects to the cladding of the fuel rods, which can lead to such defects that nuclear fuel, i.e. uranium, leaks out into the water. The debris particles can of course also cause defects on other components in the nuclear plant, for instance pumps.
With debris particles is to be understood in this application different particles such as for instance metal chips, formed in connection with various repairs of components of the plant, metal wires or other foreign particles which have entered the plant from outside, objects such as nuts, screws, smaller tools etc. Particularly difficult debris particles are such with an elongated shape, i.e. thin wires or chips which may have a length of down to about 7-8 mm. Such debris particles tend to get attached higher up in the fuel assembly, for instance in spacers. The particles vibrate in the coolant stream and may wear the cladding of the fuel rod so that a hole arises.
In order to solve this problem, it is known to provide some kind of debris filter in the lower part of the fuel assemblies, which comprise a number of fuel rods and which form the core of the reactor. One example of such a debris filter is disclosed in WO02/058075. The water, which circulates through the reactor during the operation of the plant, passes through this lower part of the fuel assemblies upwardly from below. Possible debris particles may thus be caught by one such debris filter. The debris particles are prevented at least to a certain extent from being released from the debris filter and conveyed back to the water due to the flowing of the water through the debris filter. Debris filters are normally also designed to let through smaller debris particles and in such a way reduce the risk of being stopped up. The flow through the lower part of the fuel assembly is during normal operation about 2 m/s but may at reduced power be decreased to about 1 m/s.
In connection with shutdown of the reactor, the flow of water is maintained at least initially due to the rest heat which is present in the fuel assembly. It is also possible, during an initial phase of the revision, to operate the main circulation pumps of the plant with a relatively low power sufficient for maintaining a flow of water through the fuel assemblies. Each fuel assembly is standing in a seat and water is guided during normal operation into the fuel assembly via a sealing. When the fuel assembly is to be removed and lifted out from the reactor, the flow of water will, however, be reduced and it can even be reversed depending on the speed with which the fuel assembly is lifted upwardly. This lifting speed can during a normal shutdown amount to about 1 m/s. This means that the force which is created by the flow and retains the debris particles in or immediately beneath the debris filter, is reduced or ceases and that the debris particles may fall out from the debris filter. Consequently, the tendency for the particles to be released and fall back to the reactor vessel increases. This means that there is no longer any mechanism for removing the debris particles from the reactor and the primary system, but more and more debris particles will be accumulated in the reactor vessel.
U.S. Pat. No. 5,383,226 discloses a device for handling a fuel assembly, which comprises a number of fuel rods extending between a lower part and an upper part of the fuel assembly and a casing surrounding the fuel rods. The device comprises a lifting device which is arranged during a lifting operation to engage a fuel assembly located in a reactor vessel and to lift the fuel assembly upwardly and out from the reactor vessel. A conduit member is connected to the upper part of the fuel assembly and a pump is arranged to create a flow of liquid through the fuel assembly and the conduit member to a detecting equipment. The detecting equipment is adapted to detect fission products in the water, which can indicate that one or several fuel rods are defect.